1. Field of the Invention
The present invention relates to a light scanning device used for a digital copying machine, a laser printer, a laser plotter, a laser facsimile, a laser measuring device, laser processing equipment, and the like. In particular, the present invention relates to a light scanning device having an element with which asymmetry in a curve representing the relationship between a beam spot diameter and a defocus is corrected, and an image forming apparatus, such as a digital copying machine, a laser printer, a laser plotter, a laser facsimile or a multiple function processing machine, which includes the light scanning device and is capable of forming a favorable image.
2. Description of Related Art
Recently, miniaturization of a light scanning device has been demanded along with a demand to miniaturize an image forming apparatus, such as a monochrome or color digital copying machine or a laser printer, which uses the light scanning device therein.
The reduction in variation in a beam spot diameter on a scanned surface has been also demanded along with a demand for reducing variation in dot size in an output image so as to improve the resolution of an output image.
In general, in a light scanning device used for an image forming apparatus, the following steps are performed to form a latent image: a luminous flux emitted from a light source enters a deflector (e.g., a light deflector such as a polygon mirror, a pyramidal mirror or a vibration mirror) via an optical system such as a coupling lens; the luminous flux deflected with this deflector is focused via a scanning lens, for example, and irradiated on a scanned surface (e.g., a photosensitive body or the like); and the tiny spot light beam scans the scanned surface. However, the elements of semiconductor lasers used as light sources in such light scanning devices have variation in an angle of divergence, and this variation induces a difference in the effective number of aperture stops (NA) among optical systems in the light scanning devices. As a result, there have been problems that a beam spot diameter varies among devices, or among colors in the case of outputting a color image, and that the size of a beam spot diameter is increased.
In order to avoid such problems, an aperture stop is disposed on a light path from a laser light source to a deflector, and regulates the width of a luminous flux emitted from the laser light source to guide the luminous flux with the regulated width to an image surface.
However, when the distance between a light source and a deflector is made shorter so as to miniaturize a light scanning device, a position of an aperture stop becomes so close to a scanned surface that asymmetry of in a curve representing the relationship between a beam spot diameter and a defocus (this curve is simply called “the beam spot diameter vs. defocus curve” hereinbelow) is induced as schematically shown in FIG. 5. This asymmetry has produced a problem that a variation in dot size in an output image tends to occur since the beam spot diameter is likely to vary when the beam is defocused.